Chromium electroplating began in the early twentieth or late 19th century and provides a superior functional surface coating with respect to both wear and corrosion resistance. However, in the past, this superior coating, as a functional coating (as opposed to a decorative coating), has only been obtained from hexavalent chromium electroplating baths. Chromium electrodeposited from hexavalent chromium baths is deposited in a crystalline form, which is highly desirable. Amorphous forms of chromium plate are not useful. The chemistry that is used in present technology is based on hexavalent chromium ions, which are considered carcinogenic and toxic. Hexavalent chromium plating operations are subject to strict and severe environmental limitations. While industry has developed many methods of working with hexavalent chromium to reduce the hazards, both industry and academia have for many years searched for a suitable alternative.
Given the importance and superiority of chromium plating, the most obvious alternative source of chromium for chromium plating is trivalent chromium. Trivalent chromium salts are much less hazardous to health and the environment than hexavalent chromium compounds. Many different trivalent chromium electrodeposition baths have been tried and tested over the years. However, none of such trivalent chromium baths have succeeded in producing a reliably consistent chromium deposit that is comparable to that obtained from hexavalent chromium electrodeposition processes.
Hexavalent chromium is very toxic and is subject to regulatory controls that trivalent chromium is not. The most recent OSHA rule for hexavalent chromium exposure was published in 29 CFR Parts 1910, 1915, et al., Occupational Exposure to Hexavalent Chromium; Final Rule. In this Rule, substitution is described as an “ideal (engineering) control measure” and “replacement of a toxic materials with a less hazardous alternative should always be considered” (Federal Register/Vol. 71, No. 39/Tuesday, Feb. 28, 2006/Rules and Regulations pp. 10345). Thus, there are strong government-based mandates to replace hexavalent chromium with another form of chromium. However, until the present invention, no process has been successful in electrodepositing a reliably consistent crystalline chromium deposit from a trivalent or other non-hexavalent chromium electroplating bath.
In general, in the prior art, all of the trivalent chromium electrodeposition processes form an amorphous chromium deposit. While it is possible to anneal the amorphous chromium deposit at about 350 to 370° C., and create thereby a crystalline chromium deposit, the annealing results in the formation of macrocracks, which are undesirable, rendering the chromium deposit essentially useless. Macrocracks are defined as cracks that extend through the entire thickness of the chromium layer, down to the substrate. Since the macrocracks reach the substrate, thus giving ambient materials access to the substrate, the chromium deposit cannot provide its function of corrosion resistance. The macrocracks are believed to arise from the process of crystallization, since the desired body-centered cubic crystalline form has a smaller volume than does the as-deposited amorphous chromium deposit and the resulting stress causes the chromium deposit to crack, forming the macrocracks. By contrast, crystalline chromium deposits from hexavalent electrodeposition processes generally include microcracks that are smaller and extend only a fraction of the distance from the surface of the deposit towards the substrate, and do not extend through the entire thickness of the chromium deposit. There are some instances in which a crack-free chromium deposit from a hexavalent chromium electrolyte can be obtained. The frequency of microcracks in chromium from hexavalent chromium electrolytes, where present, is on the order of 40 or more cracks per centimeter, while the number of macrocracks in amorphous deposits from trivalent chromium electrolytes annealed to form crystalline chromium, where present, is about an order of magnitude less. Even with the much lower frequency, the macrocracks render the trivalent chromium derived crystalline deposit unacceptable for functional use. Functional chromium deposits need to provide both wear resistance and corrosion resistance, and the presence of macrocracks renders the article subject to corrosion, and thus such chromium deposits are unacceptable.
Trivalent chromium electrodeposition processes can successfully deposit a decorative chromium deposit. However, decorative chromium is not functional chromium, and is not capable of providing the benefits of functional chromium.
While it would appear to be a simple matter to apply and adapt the decorative chromium deposit to functional chromium deposits, this has not occurred. Rather, for years the goal has continued to elude the many efforts directed at solving this problem and reaching the goal of a trivalent chromium electrodeposition process that can form a crystalline chromium deposit.
Another reason for seeking a trivalent chromium electrodeposition process is that trivalent chromium based processes theoretically require about half as much electrical energy as a hexavalent process. Using Faraday's law, and assuming the density of chromium to be 7.14 g/cm3 the plating rate of a 25% cathodic efficiency process with 50 A/dm2 applied current density is 56.6 microns per dm2 per hour for a hexavalent chromium plating process. With similar cathodic efficiencies and current density a deposit of chromium from the trivalent state would have twice the thickness in the same time period.
For all these reasons, a long-felt need remains for a functional crystalline-as-deposited chromium deposit, an electrodeposition bath and process capable of forming such a chromium deposit and articles made with such a chromium deposit, in which the chromium deposit is free of macrocracks and is capable of providing functional wear and corrosion resistance characteristics comparable to the functional hard chromium deposit obtained from a hexavalent chromium electrodeposition process. The urgent need for a bath and process capable of providing a crystalline functional chromium deposit from a bath substantially free of hexavalent chromium heretofore has not been satisfied.